Soil Formation

Soil Formation soil forming factors such as parent materials, climate, relief and drainage, biological activity, and the length of time, have been at work together, result in various kinds of soils being formed in Bangladesh.

Alluvial sediments of the Ganges and Brahmaputra contain different amounts of rock minerals as micas, feldspars and hornblende etc which weather in soil to produce clay and chemical elements such as Fe, Ca, P etc. For instance Tista and Brahmaputra alluvia contain large amounts of biotite (black mica), an easily weatherable mineral, but little or no lime. On the other hand, Ganges alluvium contains lesser amount of biotite and significant amounts of lime, and deposits of the Surma, Karnafuli and other rivers draining the eastern hill contain little biotite and no lime. The operation of soil forming factors on original parent material changes chemical composition, relative proportion of sand, silt and clay and secondary formation of lime and iron content.

High temperatures, high rainfall and alternately wet and dry conditions between the monsoon and dry seasons provide ideal conditions for mineral weathering in Bangladesh. Though differences in climate within Bangladesh are not so pronounced, by themselves, to make different soils in different places, but differences in rainfall at different places in Bangladesh are sufficient to create important differences in soils.

Because of differences in position on the relief, some areas of land become saturated or submerged by water for part or all of the year, whereas other places remain unsaturated throughout the year. In general organic matter decomposes more rapidly under well-drained conditions than it does in wet and anaerobic conditions. Therefore, under the conditions of Bangladesh, organic matter rapidly decomposes relatively on higher sites that are well drained, such as floodplain ridges and more slowly in depressions that remain wet for most or all of the year. Organic matter may accumulate in the form of peat in the centre of floodplain basins, creating new parent material.

Different chemical processes occur under permanently saturated, permanently non-saturated and alternately saturated and non-saturated conditions. These differences particularly affect iron compounds and this in turn affects soil colour. Grey soils occur under permanently saturated conditions where Fe2+ iron compounds are formed in the absence of oxygen and are washed out in the soil. Yellow, brown and red colours occur under non-saturated conditions where, in the presence of air, Fe3+ iron compounds are formed. Under alternately saturated and non-saturated conditions, patches of both grey and yellow-red colours are formed.

Different kinds of vegetation such as forest, grassland, cultivated crops produce different amounts and sometimes different kinds of organic matter. Soils under forest generally have more organic matter than soils on cultivated land. Again, the better drained and more permeable hill, terrace and floodplain soils are developed under forest and more poorly drained soils under grassland.

If the parent material is alluvium laid down only a few years ago (5-10 years), the changes produced by climate, vegetation and drainage are small and there is little difference between the present soil and the original alluvium. On the other hand, an alluvium laid down several hundred years ago and not subsequently buried by new alluvium, there may be considerable differences in properties between the present soil and original alluvium.

Soil formation in Bangladesh occurs under two distinct conditions: alternating seasonaly flooded or wet and dry conditions, and non-flooded conditions.

Soil formation under seasonally flooded conditions Soil formation proceeds under the following stages:

Initial condition of alluvium New alluvium is first stratified and this may be fine, as in the case of silty and clay deposits, or coarse as in the case of sandy or mixed sandy and silty deposits. Stratification in uniform silty and clay deposits is mainly caused by thin layers of mica flakes which are deposited with their flat surfaces parallel to the surface of deposit. The initial deposit is generally grey in colour. When the deposit remains wet, the individual mineral particles are surrounded by water. In this condition, the deposit is termed 'unripened'.

Ripening Ripening is the process by which the physical and chemical properties of alluvial sediments are changed by loss of the water which initially surrounds the mineral particles. This water is lost by drainage in coarse sediments, and by evaporation and plant transpiration in finer sediments. Ripening is accompanied by shrinkage of sedimentary mass and by the partial substitution of air for the interstitial water removed. The entry of air introduces chemical changes, such as oxidation of ferrous iron compounds present in the original material while it is in aerobic condition. Ripening is made apparent not only by a change in the physical condition of the material but usually in its colour, too, more particularly in silty materials.

Early development of mottles On first ripening, the soil mass has a uniform olive or brown appearance. However, stronger oxidation takes place locally where roots or animal holes allow air to enter the ripening alluvium in greater amounts. This localised oxidation is particularly strong where rice and other aquatic plants grow. That is because the living roots of such plants take down air, or release oxygen, along their channels. Water moving from the soil mass towards such roots or holes may actually move reduced iron to the interior to become oxidised in a thin zone adjoining the channel thus leaving a greyer, leached zone a millimetre or more thick behind the oxidised zone. These changes in the distribution of ferrous and ferric iron provide the first stage in the development of the colour contrasts known as mottles.

Homogenisation The process by which plant roots and soil animals disturb and mix soil material, thus eventually destroying evidence of original alluvial stratification and rock structure. The pressure created by plant roots passing through the alluvium disturb the alluvial stratification and rock layers. On their decay, plant roots leave holes through the soil mass which form soil pores through which air and water can pass. Soil animals feeding on decaying plant roots and on other soil animals further break up the stratification. By burrowing in the soil and physically mixing the material, these animals eventually destroy all signs of alluvial stratification down to the depth to which they operate.

The depth of homogenisation depends on the depth to which various soil animals feed or burrow for refuge. In alluvial materials, the lower limit may be the permanently saturated zone or coarse sand layer. In hill areas, the limit may be a hard or dense rock layer. Homogenisation is a continuous process in most soils. Not only it is responsible for mixing the original parent material to form soil material, but it also refreshes older soils by incorporating new parent material from below as weathered material is lost from the soil surface by erosion, by incorporating organic matter into the soil, and by providing new pores and voids as older ones become filled in, thus facilitating the movement of air into and through the soil.

Development of structure Loss of water from ripening alluvium causes the material to shrink both vertically and laterally. In silty and clayey alluvium, this causes cracks to develop. The first cracks that develop in thick silty material may be 5-7 cm wide at the surface and penetrate to a depth of 60 cm or more. Such initial cracks may be several meters apart in irregular patterns at first, but they eventually develop into irregular polygons. The width of polygons diminishes with age, usually to within the range of 5-25 cm in developed soils. The cracks which separate them also penetrate less deeply than the initial ripening cracks. These polygons provide the prismatic structure which is typical of most floodplain soils of suitable texture in Bangladesh.

Clay material also cracks horizontally that both prismatic and blocky structures are usually developed. Prismatic structural units may also be broken up by soil animals and roots, so that angular blocky or subangular blocky structure develops in less clayey materials.

Formation of flood coatings The formation of cracks and structural units provides holes down which water from rainfall and flooding can pass into the soil. Water also passes through holes made by roots and animals. Water passes through such holes can transport material from the soil surface and deposit it in lower layers where water movement is impeded. The sides of cracks and pores in most Bangladesh's floodplain soils are coated with such material. These coatings (called gleyans, is the name given to the shiny surface of soil cracks and pores formed by the deposition of material washed from the soil surface or topsoil under seasonally flooded condition) are uniformly grey; mid-grey if surface soil layer is grey (when wet); and dark grey if surface layer is dark grey.

The effect of these coatings is to increase the mixing of soil material by bringing topsoil material into the subsoil. They also increase the colour contrast in subsoil, for they make the outside of structural units and the walls of pores uniformly grey in comparison with the remainder of the soil mass which is usually oxidised (mottled) to varying degrees (except where this is masked by organic matter or displaced by grey earthworm casts).

Acidification and decalcification of topsoil After initial ripening, homogenisation and development of mottles, structure and subsoil coatings, the next stage in soil formation that becomes apparent is the acidification of the topsoil. Most new alluvium is near neutral to slightly alkaline in reaction when first deposited. Ganges and Lower Meghna deposits are calcareous in addition. However, unless sedimentation continues, the topsoil in noncalcareous deposits usually becomes moderately acid within about 50 years, and topsoil in calcareous deposits usually become partially or totally decalcified (leached of their lime); they may even become acid in reaction.

Changes in subsoil reaction Below the topsoil, the subsoil layers usually show little change in reaction from that of the original parent material until later stages of soil formation that generally occur in Bangladesh with the exceptions in some older piedmont soils, in acid sulphate soils, in some decalcified old ridge soils on the Ganges River Floodplain, and in the Acid Basin Clays and in some Calcareous Dark Grey Floodplain Soils on the Ganges River Floodplain.

Ferrolysis- Initial stage Under the reduced conditions that exist in floodplain topsoil when they are submerged by flood, ferrous iron is formed and displaces cations from the clay humus complex, from where they are removed in the floodwater. On reaction of the topsoil after the floodwater recedes, the iron changes to the ferric form again and is replaced in the exchange complex either by cations released by mineral weathering or, where such weathering dose not release, these cations are replaced by hydrogen and aluminium, in which case the soil reaction becomes acid.

In case of calcareous material the first stage in the ferrolysis is that of decalcification. This takes place rapidly under reduced condition where the decomposition of organic matter increases the partial pressure of CO2, allowing Ca(HCO3)2 to be formed from the CaCO3 present. The bicarbonate is dissolved by the floodwater and carried away by it. Once the topsoil has been decalcified, acidification by ferrolysis can begin.

The topsoil reaction, in fact, changes seasonally with the change between reduced and oxidised conditions. Under reduced conditions, the topsoil is near-neutral in reaction, even in calcareous material. On aeration the topsoil becomes acid in reaction in noncalcareous material and alkaline in calcareous material. However, the topsoil of Black Terai soils does not become reduced when flooded, or are not partially reduced, so that topsoil in these soils remain acid even when submerged.

Oxidation of subsoil The seasonal change in oxidation reduction only affects, the topsoil except perhaps where buried organic layers occur. The subsoil of most floodplain soils remain aerated throughout the period of submergence, even when the topsoils are deeply submerged. This seems to be due to the presence of air entrapped in subsoil pores and cracks when these voids are sealed off from the surface by ploughing or puddling of the topsoil.

The fact that floodplain subsoil generally remains aerated throughout the year, has three implications. The first is that mineral weathering continues under aerobic conditions. Since all floodplain sediments are rich in iron bearing minerals some of which, like biotite, are easily weathered- the subsoil rapidly develop oxidised iron colours. The second implication of continuous subsoil aeration is that most floodplain soils, except where sandy and in fine structured Gangetic clays, do not have a water table, at least as conventionally understood, ie, as a continuous body of free moving water within the soil mass. A third implication of the continuous subsoil aeration is that conditions remain suitable for the survival of air-breathing soil animals in most soils despite the soil being submerged

Differentiation of topsoil and subsoil texture The continuation of ferrolysis eventually leads to the strong weathering of minerals in seasonally flooded topsoil. Weatherable sand and silt minerals such as biotite are attacked, breaking them down to finer silt and clay minerals. Huizing gives figures showing reductions in the ratio of biotite to total micas from about 0.65-0.75 in subsoil and substratum of Brahmaputra-Jamuna and in Tista alluvium from 0.45-0.50 in the topsoil of soils on so-called young floodplain and to as low as 0.10-0.35 in old floodplain soils. A Shallow Grey Terrace Soils examined had a ratio of 0.20 in the substratum and 0.10 in the strongly ferrolysed topsoil and subsoil. Clay minerals appear also to be destroyed or altered. The result of this mineral weathering in topsoils that are markedly coarser in texture than the underlying subsoils.

Differences in texture between topsoil and subsoil are found in some Noncalcareous Grey Floodplain Soils on Jamuna Floodplain formed in deposits that are less than 200 years old. On the Old Brahmaputra Floodplain and Old Meghna Estuarine Floodplain which are more than 200 years but probably not more than 1-2000 years old, the prevalent Noncalcareous Dark Grey Floodplain Soils commonly have difference in clay content of 5-15 percent in topsoil and subsoil, and differences of 20-30 percent are found in some Acid Basin Clays. In such soils, evidence of degradation is often present in the form of white silt specks and partings in the ploughpan, and as silt coatings along cracks and pores in the upper pan of subsoil.

Formation of ploughpan Human activities have accelerated the process of ferrolysis in cultivated soils. Tillage with flat-soled plough in general use in Bangladesh compacts a layer at the base of the topsoil, forming a dense, impervious ploughpan in most soils. These compact layers impede internal drainage, leading to surface waterlogging. The cultivated layer above the ploughpan becomes more strongly reduced under these conditions than in uncultivated soils, leading to more rapid ferrolysis.

Accumulation of organic matter Alluvial deposits in depression which stay wet throughout the year do not ripen. Where they remain in this state for a long time, organic matter gradually accumulates on the surface, derived from decaying aquatic vegetation or from the residues of crops such as boro rice that may be grown in such sites. This organic matter provides a darker colour to the topsoil. It also causes strong reduction of iron released by mineral weathering. This may give a bluer or greener colour to the soil material where it is not masked by the presence of large amounts of organic matter.

In the older depressions, organic matter may accumulate on the surface in the form of peat or muck. Under the special conditions of tidal mangrove forest, sulphur accumulates in the organic matter contained in unripened alluvium, creating potentially toxic soil conditions.

In soils that dry out seasonally, the organic matter content is related to the duration of wet conditions and to the age and use of the soils. The longer the period of seasonal flooding, the shorter the period during which rapid aerobic decomposition of organic matter can take place. Therefore, in general, basin soils tend to contain more organic matter than associated ridge soils.

Ferrolysis: later stages' There is a gap in the history of soil formation between the oldest floodplain soils, and the seasonally flooded terrace soils. No floodplain soils have been progressed further than the degradation of the topsoil and the development of thin white silt skins in the upper subsoil, but extensive poorly drained areas on the Barind and Madhupur tracts, and on parts of the piedmont plain in the north of Mymensingh region, have soils in which degradation extends to depth of 30-60 cm or more. These are the Shallow and Deep Grey Terrace Soils, and Grey Valley Soils. In these soils, degradation is specially visible in the topsoil. This layer may have 20-40 percent less clay than in parent Madhupur Clay; the subsoil may also have 10-25 percent less clay than underlying substratum.

Strong mineral weathering and clay destruction have taken place in these soils as a result of long-continued ferrolysis. This process may have originated under natural grassland several thousand years ago, but ferrolysis has been intensified in the topsoil as a result of puddling for transplanted rice cultivation.

Occurrence of non-reduced topsoil Some floodplain ridge soils which are seasonally flooded do not become reduced in the topsoil when the soils are submerged. This happens in sandy materials, such as in Black Terai Soils, where tillage even for transplanted rice cultivation does not form a complete puddled topsoil and a strong ploughpan. It also occurs in soils on high floodplain ridges which are flooded only intermittently for less than 14 consecutive days which are required for reduction to occur. In such soils, water flowing over or moving through the soils maintains aerated conditions throughout the profile down to the water table. Soil formation in these soils takes place under aerated conditions.

Salinization Some land near the coast is tidally flooded with salt water for part or all of the year producing some soils. Such soils are most extensive in the southwest of the Ganges Tidal Floodplain, in coastal areas of the Young Meghna Estuarine Floodplain and on tidal floodplains within Chittagong Coastal Plain. Small patches also occur locally on ridge tops in some western parts of the Ganges River Floodplain, mainly in Jessore regions. In most areas, the salinity is caused by capillary movement of water to the surface from saline groundwater, which is present even in areas protected by embankment.

Alkalisation The long continued capillary movement of salts from the permanently saturated zone to the surface can eventually lead to sodium replacing more than 15 percent of the exchangeable bases in the topsoil. This causes the soil material to disperse when it is wet. The topsoil then becomes very compact and impervious, which prevents the downward leaching of soluble salts in rainy season. Such layers become strongly alkaline, with pH values of 9-11, which makes them unsuitable for cultivation under traditional agriculture. Small patches of alkali soils have been encountered only in the western part of the Ganges River Floodplain mainly in Jessore region, where the floodplain deposits may be several thousand years old (forming part of the so-called moribund Ganges delta).

Calcification When first deposited, Ganges river alluvium contains about 5-10 percent of lime (Calcium carbonate) mainly in silt and fine sand fractions. During soil formation, this lime is subject to leaching. Most of it is removed from the soil, but there is often a little redistribution within the profile, made evident by the occurrence of a few, small, hard, irregular, lime nodules, apparently formed along old root channels or in infilled small animal holes. In two areas, lime has been redistributed in different forms. In one case, on the oldest part of the Ganges River Floodplain in west of Kushtia and Jessore regions, some ridge soils occur in which the topsoil and a part or all of the subsoil have been leached of lime but small amounts of solid powdery limes are visible in the subsoil or substratum. The other area where soils with lime accumulation occur in central Rajshahi region, where Atrai river deposits occur alongside the Ganges River Floodplain. Here an extensive area of soils has a thick layer of hard, irregular, lime nodules in the subsoil or substratum, amounting to about 20 percent of the soil (by weight).

Soil formation on non-flooded conditions Under these conditions, the most significant processes are homogenisation, oxidation, leaching and mineral weathering, and in some soils erosion is also important. Soils in which these processes are dominant occur on high floodplain ridges and well-drained terrace land as well as hill areas. Similar processes occur in soils that are seasonally flooded but in which the topsoil does not became reduced such as Black Terai Soils.

High floodplain ridge soils on floodplain ridges that are flooded only for short periods or during exceptionally high floods, new alluvium is rapidly homogenised and oxidised except in very sandy materials where these processes operate more slowly. Homogenisation by plant roots and animals breaks up stratification, usually down to a coarser sandy layer or permanently saturated zone in the substratum. High temperature, rainfall and humidity of Bangladesh allow minerals to weather relatively rapidly. Because most kinds of floodplain alluvium contain large amounts of biotite, therefore is a considerable release of iron, which under the aerated conditions provides oxidised soil colours. High floodplain ridges invariably consist of permeable sandy or silty materials, and these are made even more permeable by vigorous biological activity. Rainwater falling on the soil is mainly absorbed and passes through the soil. Carbon dioxide absorbed from the atmosphere in the rainwater provides a dilute solution of carbonic acid, which causes leaching of bases and calcium carbonate.

These processes result in the formation of Calcareous and Non-calcareous Brown Floodplain Soils on high floodplain ridges. These soils have a uniform brown, olive-brown or yellow-brown subsoil colour. The noncalcareous soils have invariably been leached: pH values usually are 5.0-6.0 in the topsoil and subsoil; and 6.0-7.0 in the substratum.

Himalayan Piedmont Plain soils Soil development has apparently been in progress for several thousand years on the Himalayan Piedmont Plain. In the most widespread soils, the profiles are homogenised and oxidised down to about 90-l20 cm, below which there is grey or white loose sand. If this sand is same as that which formed the parent material of the overlying soils, there must have been considerable weathering of sand minerals to produce the higher amounts silt and clay which the upper layers contain (generally 10-20 percent higher than in the substratum). However, mineralogical studies show that there is little difference in the degree of weathering of biotite between substratum and the upper soil layer, so it seems probable that the textural difference between these layers reflects an original difference in texture within the parent alluvium.

Soils of the Himalayan Piedmont Plain have dark coloured topsoil, specially in the north where they are recognised separately as Black Terai Soils. Organic matter has accumulated in unusual amounts and to unusual depth in these soils because they lie wet, though not reduced, during the monsoon season and because there is strong biotic mixing. Black topsoil range from 25 cm to more than 75 cm thick in the Black Terai Soils. In Noncalcareous Brown Floodplain soils, the topsoils are of a similar thickness but gradually become less dark usually dark brown as soil conditions become less saturated in the monsoon season (probably partly due to lower rainfall, partly to better soil drainage on the more pronounced relief).

Hill soils The processes of homogenisation, oxidation and leaching also occur in materials released by weathering and erosion in the Northern and Eastern Hills, giving rise to the Brown Hill Soils. Because of the higher rainfall and lack of groundwater influence, these soils are more acid in reaction, usually pH 4.0-4.5 throughout the profile. The rocks from which these soils have developed disintegrate rapidly under the influence of percolating rainwater and air, assisted by disturbance aided by deeply-penetrating tree roots. Mineral weathering releases iron as well as some clay, so the soils which are formed become more oxidised and clayey than their parent rocks. There is very strong biotic activity in these soils, both by wide ranging roots and by soil animals, especially earthworms and termites. The resulting homogenisation makes the soils uniform in texture and colour in the topsoil and subsoil. Most hill soils are strong brown or yellow-brown and loamy; a few soils derived from unconsolidated sandstone are redder, and some from shales are greyer.

Brown Hill Soils mainly occur on steep slopes. Removal of material from the soil surface by erosion apparently keeps pace with the formation of soil material by weathering of the parent material, except on the steepest slopes and where the forest cover has been destroyed to make way for cultivation. In general, the soils are about 60-90 cm thick, but they are thinner on some very steep slopes and over some shaley rocks where erosion is more rapid, and they are thicker on gentle slopes and over more sandy rocks. The combined effect of constant removal of material from the surface and incorporation of new material from below preserve the soils in a relatively young stage of development.

Clay skins (argillans) are present in the subsoil and substratum of some hill soils, reflecting the downward leaching of clay from the topsoil into lower layers. In general, these coatings seem to be present in small quantities and they are fragmented, presumably due to biotic mixing of the soil material.

Terrace soils In some upland areas, the removal of material by erosion is slower than incorporation of new material from below by weathering of parent material. This is the situation in some level and gently sloping hill areas, on level, well-drained areas of Madhupur and Barind Tracts, and on some areas of latter tracts that were formerly level but have subsequently been dissected by valleys. Weathered material accumulates over the parent rock, therefore, giving rise to Deep Red- Brown Terrace Soils. They generally are less acid than the hill soils, usually with the pH values in the range 5.0-5.5, or occasionally higher.

On Madhupur and Barind tracts, weathering of parent Madhupur Clay started many thousand of years ago, under different climatic and geomorphological conditions from the present. For instance, the large amounts of lime nodules and the presence of silikensides found in some shallowly weathered soils are thought to have been inherited from drier and possibly hotter climatic conditions several thousand years ago.

In shallow terrace soils, the impervious heavy Madhupur Clay substratum has impeded soil development. In first place, the depth of homogenisation is limited because the penetration of roots and soil fauna are impeded by the compact nature of the swelling clay, its wet condition in the rainy season and its very dry condition in the dry season. Additionally, the heavy clay impedes internal drainage, causing runoff of excess rainwater to occur, taking with it material from the soil surface.

In deep terrace soils, weathering in the substratum has taken place under the influence of a seasonally fluctuating watertable, producing red mottles in the substratum. Strong biotic activity extending into this layer and bringing up material to the surface have produced a subsoil that is uniformly red (or brown on less well-drained sites). Biological activity under the original natural forest also mixed organic matter deeply into the profile, although the colour of this is masked by strong oxidation colours. Despite the biotic mixing, clay has apparently been lost from the topsoil to the subsoil and substratum in these soils, so that the topsoil is lighter in texture than these subsurface layers. Illuvial clay skins are visible in some soils, but thin section studies suggest that they may be fossil because they are often fragmented, presumably by biotic mixing, and appear to have been preserved mainly in oxidised parts of the red mottled substratum.

Mineralogical studies show that the Deep Red-Brown Terrace Soils of Madhupur and Barind Tracts are not completely weathered. About 40 percent of the clay fraction are made up by illite and vermiculite, and relatively unweathered alkali and plagioclase feldspars remain in the sand fraction. Dark coloured minerals have been almost completely lost by weathering, although it must be remembered that the original content of such minerals in the parent Madhupur Clay is small, too. [Aminul Islam, Sirajul Hoque and Md Nazmul Hasan]

Bibliography FNE Martinez Ponnamperuma and T Loy, 'Influence of redox potential and partial pressure of carbon dioxide on pH values and suspension effect of flooded soils', The Journal of Soil Science (6), 1966; AKM Habibuilah and DJ Greenland, 'Clay mineralogy of some seasonally flooded soils of East Pakistan (Bangladesh)', The Journal of Soil Science (2), 1971; Technical Report-3, Soil Survey Project of Bangladesh - 1971, Based on the work of Hugh Brammer; SM Saheed, 'Soils of Bangladesh', Proceedings of the Inter Congress Meeting of Commission IV, International Society of Soil Science, 1984; Hugh Brammer, The Geography of the Soils of Bangladesh, UPL, Dhaka 1996.